Compressor and turbine rotors (or wheels) as well as centrifugal impellers used in gas turbine engines represent load bearing components which would have an equiaxed fine grain microstructure in the hub-to-rim regions for optimum low cycle fatigue resistance at service temperature and an equiaxed cast grain, directionally solidified columnar grain, or single crystal structure in the blades for optimum high temperature stress rupture strength at service temperature.
Although integrally cast bladed turbine rotors have been successfully used for years in many small turbine engine applications, the prior art has recognized that the conventional investment cast rotor inherently compromises the ideal microstructure described in the preceding paragraph. Namely, the relatively massive hub section of the casting exhibits a coarse, columnar grain structure due to its slower solidification and cooling after casting, while the rim section may exhibit a finer, columnar grain structure. As a result of their thin section, the integrally cast blades exhibit a generally equiaxed, finer grain structure sometimes including columnar grains with an unsatisfactory orientation. The significance of such a compromise in the microstructure of the turbine rotor becomes apparent when it is recognized that the mechanical properties of the casting are a function of the number and orientation of the grains in the particular region of interest. For example, coarser grain structures are known to offer better elevated temperature stress rupture properties than a fine grain structure. However, the latter grain structure offers better low cycle fatigue properties. Moreover, the low cycle fatigue properties within a cast component depend on the crystallographic orientation of grains relative to the local distribution of stress(es). An unfavorably oriented coarse, columnar grain in a conventionally cast component can contribute to premature failure of the component.
An improved investment casting process, known as the Grainex.RTM. investment casting process, was developed to enhance the uniformity of the microstructure of integrally cast bladed rotors (specifically integral turbine wheels) to meet new challenges of component performance and reliability demanded by increased thrust and horsepower applications. The Grainex process includes motion of the mold during solidification of the melt and also, a post-casting HIP (hot isostatic pressing) treatment. This process develops a substantially uniform fine, equiaxed grain structure through the hub, web and rim regions of the casting. This microstructure provides a significant improvement in consistency in the low cycle fatigue properties in these sections of the cast turbine wheel while providing stress rupture properties in the blades similar to those obtainable in conventionally investment cast integrally bladed rotors.
Another improved investment casting process, known as the MX.RTM. investment casting process, also was developed to enhance the uniformity of the microstructure of castings. The MX process involves filling a properly heated mold with molten metal having little superheat (e.g., within 20.degree. F. of its measured melting temperature) and then solidifying the molten metal in the mold at a rate to form a casting having a substantially equiaxed cellular, non-dendritic microstructure uniformly throughout with attendant improvement in the mechanical properties of the casting. U.S. Pat. No. 4,832,112 describes this process.
Integrally bladed rotors also have been fabricated by machining processes which utilize either ingot or consolidated metal powder starting stock. The powder metal rotors are generally consolidated by hot isostatic processing (HIP) and demonstrate reduced alloy segregation compared to ingot metallurgy. Powder metal rotors are, however, susceptible to thermally induced porosity (TIP) from residual argon used in powder atomization. Any oxygen contamination of powders can form an oxide network resulting in metallographically detectable prior particle boundaries which are known sites of fracture initiation. These limitations make manufacture of rotors by machining of ingot or consolidated metal powder costly in terms of both processing and quality controls.
Advanced powder metal manufacturing and consolidating techniques coupled with advanced forging processes have provided the capability to produce fine grain rotors which exhibit improved low cycle fatigue properties as compared to conventional investment cast rotors. However, the forged rotors typically exhibit inferior stress rupture properties in the rim compared to conventional investment cast rotors.
Unfortunately, in general, metallurgical processing to maximize low cycle fatigue properties of a metal results in reduced creep (stress rupture) properties. As a result, in more demanding service applications where increased thrust and horsepower are required (e.g., in military aircraft), designers have often resorted to the traditional separately bladed/mechanical attachment approach that involves fabricating a fine-grained, forged disk; machining serrated slots in the disk to accept machined blade roots; and inserting cast blades of the desired grain structure (e.g., directionally oriented or single crystal) into the slots. However, machining slots and blade roots are costly processing steps. This method also limits the number of blades that can be attached, especially in smaller engines. A design with a large number of blades often is desirable for higher performance.
Those skilled in the art of turbine engine design have recognized the potential advantages of combining the ease of fabrication and the structural integrity of monolithic integrally cast/forged rotors with the high performance capability obtainable in separately bladed turbine engine rotors. Several approaches have been developed to produce such a turbine rotor. One such approach is illustrated in U.S. Pat. No. 4,096,615 wherein an equiaxed blade ring is cast and then solid state diffusion bonded to a separately produced powder metal hub or disk in a hot isostatic pressing step. Both an interference fit and braze sealing usually are required to achieve complete bonding during HIP'ing. In particular, a radially inwardly facing surface of the blade ring is machined to precise diameter to form a bonding surface adapted to mate with the radially outwardly facing bonding surface of a hub or disk made of another material. The blade ring is positioned over the hub and oxygen and other contaminants are removed from the bonding surfaces by vacuum treatment, followed by braze sealing the external joint lines with braze material. Hot isostatic pressing then is used to diffusion bond the blade ring to the hub. This approach has the disadvantage of requiring several separate processes: (1casting the blade ring; (2) precision machining the inner diameter of the blade ring; (3) powder metal HIP consolidation; (4) precision machining the outer diameter of the powder metal hub, (5) assembly of the blade ring and powder metal hub; and (6) a second HIP operation to achieve final solid state diffusion bonding. Each of these processes is expensive and may create additional costs arising from defect scrap losses. Moreover, the braze sealing operation has the potential to seep braze alloy into any gap at the interface and cause a localized embrittlement or weakness in the joint.
U.S. Pat. No. 4,270,256 describes a somewhat similar process for making a hybrid turbine rotor wherein an expendable blade fixturing ring is used to position the blades for bonding directly to a hub in a hot isostatic pressing step. The blade fixturing ring is removed after the blades are bonded to the hub.
A similar, complex approach for manufacturing a dual-alloy integrally bladed rotor is illustrated in U.S. Pat. No. 4,529,452. In that approach, a blade ring is formed by diffusion bonding a plurality of single crystal elements together. The bonded blade ring is then bonded to a hub by a superplastic forming/solid state diffusion bonding step.
Another approach used in the art employs powder metal in an investment mold which has directionally solidified or single crystal cast blades positioned within it. The mold is loaded in a metal can, covered with an inert pressure-transmitting media, vacuum sealed and hot isostatically pressed. This combined blade/powder metal approach has less process steps than the interference fit approach described immediately above but is severely limited in dimensional control due to blade/mold movement during subsequent consolidation of the 65-70% dense powder.
A relatively new low pressure, high velocity plasma spray method to produce fine grain, load-bearing structural components (as opposed to protective coatings on a component) is illustrated in U.S. Pat. Nos. 4,418,124 and 4,447,466. This low pressure, high velocity plasma spray method to produce structural components employs a spraying procedure described in U.S. Pat. No. 3,839,618. Attempts have been made to use the low pressure, high velocity plasma spray technique to fabricate dual alloy turbine wheels. In these attempts, a plasma gun in a dynamic partial vacuum (low pressure) is used to plasma spray molten metal onto a solid metal substrate in the form of an integrally bladed dish-shaped member. In particular, metal powder feedstock is injected into the plasma gun and propelled to the substrate in a carrier gas. A plasma jet deposits molten droplets of the spray cast metal on the surface of the solid substrate where the droplets solidify incrementally until the desired structural shape (e.g., a rotor hub preform) is obtained. The droplets are deposited by line-of-sight to produce simple near-net-shape configurations with a joint between the initial solid substrate (e.g., investment cast substrate) and the spray cast metal deposit. The spray cast deposit can be different in composition and/or microstructure from the initial solid substrate. After deposition of the spray cast metal, the preform is hot isostatically pressed (i.e., HIP'ed) to substantially eliminate voids primarily in the spray cast metal and perfect bond the spray cast metal and solid substrate at the bond joint therebetween.
It is an object of the invention to provide a method of making a dual alloy article wherein a plurality of individual components, such as airfoils, having a directionally oriented (solidified) or single crystal grain structure are joined together in a manner to form a ring/container into which molten metal is plasma sprayed to form a dual alloy article that is amenable for subsequent hot isostatic compaction.